1. Field of the Invention
The present invention relates to Multi-Input Multi-Output Orthogonal Frequency Division Multiplexing (MIMO-OFDM) for communication using a plurality of antennas and a plurality of subcarriers, and also relates to the technique of a high-speed wireless LAN.
2. Description of the Related Art
A conventional wireless LAN (IEEE 802.11a) transmits known symbols (short preamble, long preamble) in advance of a data signal so as to perform synchronous processing and channel estimation and can demodulate a data part by using these preambles. MIMO is known as a technique to improve a transmission speed without expanding a frequency band width. Since a transmitting/receiving device in MIMO uses a plurality of antennas, MIMO has to change a structure of the short preamble and the long preamble in order to apply MIMO to the conventional wireless LAN.
According to a preamble structure idea proposed in a document 1 (Jan Boer, et al. two “Backwards compatibility”, September, 2003, IEEE LMSC, the Internet <URL:ftp://ieee:wireless@ftp,802wirelessworld.com/11/03/11-03-0714-00-000n-backwards-compatibility.ppt>), at first, a short preamble sequence used for time synchronization, frequency synchronization and automatic gain control (AGC) is transmitted, a long preamble including symbols for channel response estimation and a signal field are transmitted from one transmitting antenna. Then, the long preambles for the channel response estimation are transmitted in order. Thus, data is simultaneously transmitted from a plurality of transmitting antennas after transmitting the preamble signals. That is, the long preambles for the channel response estimation are transmitted by using the plurality of transmitting antennas through time division multiplexing.
By the way, in a MIMO system, a receiver has to acquire information about the number of transmitting antennas, a modulation method, an encoding rate, etc. so as to demodulate a received signal. For the purpose of acquiring the information on the receiver, a system for transmitting a dedicated signal to inform the information is taken into consideration. But the system has problems of increase in overhead and deterioration in throughput. In the case such that information needed for demodulation is estimated from the received signal, the demodulation becomes impossible if the estimation is resulted in failure, so that the estimation requires high accuracy. A system for estimating the information using the received preamble signals is assumable, however, the preamble signals as described in the document 1 is not premised on the estimation of the information, so that it is hard to accurately estimate the information by using the preamble signals.
In the MIMO system as just mentioned, in the case of the estimation of the information (such as the number of the transmitting antennas, etc.)necessary for the demodulation from the received signal, there is a problem such that the MIMO system can not easily estimate the number of the transmitting antennas with high accuracy, etc. by using the preamble signals in the document 1, because the estimation requires accuracy higher than that of for demodulation of data. And, the system in the document 1 for transmitting the information necessary for the demodulation involves the problem such as the increase in overhead.
Therefore, an object of the invention is to easily estimate the notification item necessary for demodulating data symbols on the receiver and correctly demodulate the data symbols without having to add the dedicated signal to inform the notification item.